1. Field of the Invention
The present invention relates in general to exhaust gas purifying systems of an internal combustion engine and more particularly to exhaust gas purifying systems of a type which has a NOx-occluded type three-way catalytic converter installed in an exhaust passage extending from the engine. More specifically, the present invention is concerned with a control system employed in the exhaust gas purifying system of the above-mentioned type for controlling an air/fuel ratio of a mixture to a level appropriate for effectively purifying NOx in the exhaust gas.
2. Description of the Prior Art
Internal combustion engines having a NOx-occluded type three-way catalytic converter installed in an exhaust passage have been known, one of which is described in Japanese Patent First Provisional Publication 7-139397.
As is known, the NOx-occluded type three-way catalytic converter has such a performance that when the exhaust gas from the engine shows a higher air/fuel ratio (viz., lean), the converter occludes NOx in the exhaust gas, while when the exhaust gas shows a stoichiometric or lower air/fuel ratio (viz., rich), the converter releases NOx allowing reduction of the same with the aid of HC and CO in the exhaust gas.
For ease of description, the NOx-occluded type three-way catalytic converter will be referred to as NOx-occluded converter in the following.
That is, under lean combustion of the engine, the NOx-occluded converter occludes NOx thereby reducing NOx in the exhaust gas actually discharged to the open air. However, when the amount of NOx occluded by the converter comes up to a maximum occluding capacity of the converter, NOx in the exhaust gas from the engine would be directly discharged to the open air without being occluded by the converter. For dealing with this undesired phenomenon, a so-called "rich spike control" has been proposed wherein when the amount of NOx occluded by the NOx-occluded converter is determined to come up to the maximum occluding capacity, the target air/fuel ratio of a mixture fed to the engine is temporarily changed to a lower side (viz., richer side) to force the converter to release NOx for reduction of the same. Usually, the determination of the maximum occluding capacity of the converter is made based on an engine load, an engine speed, an air/fuel ratio of the mixture, etc.